This invention pertains to the brazing of metals and, more particularly, to improvements in the brazing of particles of aluminum and its alloys to form a shaped porous aluminum body, and to bodies so formed.
Heretofore metallic porous bodies such as utilized, for example, in porous plug burners and porous plug heat exchanger systems such as shown in U.S. Pat. No. 3,750,399 and U.S. Pat. No. 3,740,313, respectively, have typically been made by fusing together tin-plated copper pellets according to conventional methods. Another well known application for such porous bodies is a filter body for liquids such as gasoline. However, because of the costliness of materials for such bronze porous bodies and because, in some applications, weight may be an important consideration, it is desirable to form such bodies of aluminum. In the past, attempts to do this have not been successful because of the tenacious layer of oxide that coats any aluminum surface exposed to oxygen in any quantity.
Generally, the soldering or brazing of aluminum bodies has heretofore been typically done either in air using a flux or in a vacuum using an active material, which active material doesn't require a separate flux to permit the formation of a bond between the aluminum bodies. Both such techniques are old and well known for large, non-porous aluminum body constructions. In both of these techniques, the tenacious aluminum oxide is removed by chemical reaction which allows the solder to flow over and bond to a clean aluminum surface.
When the soldering or brazing of large body assemblies has been done in air, the removal of chemical by-products and the formation of fillets has been relatively easy. However, when such techniques have been applied to aluminum pellets, difficulty has been encountered because the flux and solder completely fill the majority of the spaces or interstices between the aluminum pellets. One reason for this is thought to be that, in air, an excess of flux is required to allow the particles to bond, which excess causes the solder to flow and fill the interstices.
Fluxes typically used in aluminum brazing include highly corrosive boron and fluorine compounds. For this reason, techniques which include the use of such fluxes for brazing aluminum bodies in a substantially inert environment likewise have generally been found to be unacceptable because of the difficulty in removing the chemical by-products and the reactive metal vapors released by the chemical reactions between the flux and the aluminum bodies. These metal vapors are detrimental to the formation of a body of high structural integrity as well as being detrimental to the vacuum chamber itself if they are improperly distributed or filtered off from the brazing chamber.
Fluxless brazing techniques have also been found to be unacceptable for constructing porous bodies of small aluminum pellets for similar reasons. The removal or distribution of the corrosive gas vapors, also including water vapor, which are created or released during the brazing process, is made extremely difficult because of the necessity of enclosing the aluminum pellets within a tightly closed mold for shaping of the body to be formed.
"Inert environment" is used herein to refer to inert gas atmospheres and to vacuum or partial vacuum atmospheres. The critical factor in this definition is the fact that the "inert environment" be substantially inert with respect to aluminum; i.e., the environment has a reduced amount of oxygen -- either as free oxygen or in a compound such as water -- relative to the amount normally contained in the air.
It is an object of this invention to provide a porous body of aluminum pellets.
It is another object of this invention to provide a method for forming porous aluminum bodies.
It is still another object of this invention to form a porous aluminum body in a substantially inert environment utilizing a minimum of flux.
It is yet another object of this invention to provide a method for forming a porous aluminum body whereby excess brazing material does not deleteriously affect the bonding of aluminum pellets in a porous configuration.
These and other objects will be apparent from the following description taken in conjunction with the appended claims.